I upgraded my computer to Windows 10 on 30 July 2015. While Galactic Civilizations 3 seems to run fine, I noted that my gave saves were not working (Manual or Auto). There is no direct indication (e.g. error message) that the saves do not occur. When starting the game later (e.g. the next day) the saved games listed are the ones last saved prior to the Windows 10 upgrade (i.e. 29 July).
I uninstalled and reinstalled GalCiv 3 twice, but the problem remained. I have even started a new game (which works just fine), but cannot save it even from the beginning.